The present invention relates generally to digital image enhancement and more specifically to a method of contrast mapping using a variable mask.
When taking photographs of an outdoor scene, the brightness ratio, or dynamic range, of an area in full sunlight to another area in deep shade can exceed 1000:1. When a reflective print is created of this scene the print is typically limited to a dynamic range less than 100:1. Because of the large mismatch between the dynamic range of the reflective print and the original scene, there needs to be some mapping or transformation of the dynamic range of the scene onto the available dynamic range of the reflective print to create a pleasing reproduction. A common technique used to map one dynamic range onto another dynamic range is the use of a nonlinear curve, often implemented digitally with a tone map. Tone maps are lookup tables that transform numbers from an input range (1000:1 in an outdoor scene) to a new range (100:1 for a reflective print). Tone maps are typically applied globally; all points in an image that have a certain numerical value are mapped to precisely the same output value. Unfortunately when using a tone map to map a large dynamic range onto a much smaller dynamic range the resulting image often produces a sensation of reduced contrast. This reduction in contrast makes the image appear flat and unappealing.
Photographers have addressed this problem by darkroom manipulations called burning and dodging. Dodging is a technique, performed while printing a negative onto photographic paper, in which dark areas of the image are lightened by casting a shadow over the area for some portion of the total exposure time. The shadow reduces the amount of light exposure to the print thereby lightening that part of the image. The shadow is commonly produced by blocking light projected by the enlarger lens with a piece of cardboard on a piece of wire, or even with the photographer""s hand. Burning is the opposite process in which additional exposure time is given to the photographic paper by increasing the amount of light exposure to some area of the print. Typically a card with a small hole is held over the print with the light falling through the hole exposing only the area of the image to be darkened.
Dodging and burning work well for large areas of slowly changing brightness, but produce halos in areas that have complex shapes.
Another technique used to reduce the dynamic range in a scene is the Retinex algorithm developed by John McCann of Polaroid. Retinex is based on the Retina and Cortex theory of human vision developed by Edward Land. The basic Retinex algorithm is described in U.S. Pat. No. 4,384,336 xe2x80x9cMethod and Apparatus for lightness imagingxe2x80x9d which is hereby incorporated by reference. The Retinex algorithm reduces the dynamic range of an image by removing much of the slowly varying changes in illumination in the scene. For images that have slowly changing illumination the original Retinex algorithm can make significant improvements in the appearance of the mapped reflective print. For images that have areas with rapidly changing levels of illumination, or even abrupt boundaries between different sources of illumination, the original Retinex algorithm may only produce subtle improvements.
What is needed is a method that reduces the large brightness ratios, or large contrasts, and at the same time preserves, the small, local brightness ratios in the image. This is important to preserve the sensation of contrast when viewing the image. Therefore there is a need for a system that can compresses large contrast differences between different areas of an image while preserving small contrast differences between different areas of an image.
A method of local contrast mapping that changes the dynamic range of an original image to more closely match the dynamic range of the medium used for the reproduction. The method modifies the contrast differences between different areas of the original image as a function of the distance between the different areas. Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.